Brake force control apparatus

ABSTRACT

In a brake force control apparatus selectively performing a brake assist (BA) control for generating a brake force greater than that of a normal control time, an unnecessary restart of the BA control immediately after the BA control is terminated is prevented. The BA control is terminated when it is determined that an emergency braking is unnecessary since a master cylinder pressure (PM/C) has been reduced to a value less than a predetermined value. A restart of the BA control which is caused by a sharp change in the master cylinder pressure (PM/C) and a rate of change (ΔPM/C) thereof due to a rapid decrease in a fluid pressure when the BA control is terminated is prohibited until a predetermined time has been passed after the BA control was terminated.

TECHNICAL FIELD

The present invention relates to a brake force control apparatus and,more particularly, to a brake force control apparatus which generates,when an emergency braking is required, a brake force greater than thatgenerated in an ordinary time.

BACKGROUND ART

Conventionally, for example, as disclosed in Japanese Laid-Open PatentApplication 4-121260, a brake force control apparatus which generates,when an emergency braking is required, a brake force greater than thatgenerated in a normal time is known. The above-mentioned conventionalapparatus comprises a control circuit which generates a drive signalcorresponding to an operational speed of a brake pedal and a fluidpressure generating mechanism which generates a brake fluid pressurecorresponding to the drive signal generated by the control circuit.

The control circuit determines that, when an operational speed of abrake pedal is less then a predetermined value, the brake pedal is notnormally operated. In this case, the fluid pressure generating mechanismis controlled so that a brake fluid pressure corresponding to a brakepressing force is generated. Hereinafter, this control is referred to asa normal control. Additionally, the control circuit determines that,when an operational force of the brake pedal exceeds a predeterminedvalue, an emergency braking is required by the driver. In this case, thefluid pressure generating mechanism is controlled so that a brake fluidpressure is maximized. Hereinafter, this control is referred to as abrake assist control. Thus, according to the above-mentionedconventional apparatus, a brake force corresponding to a brake pressingforce can be generated in a normal time, and a large brake force can beimmediately generated in an emergency.

In the above-mentioned conventional apparatus, a normal brakingoperation and an operation requiring an emergency braking arediscriminated in accordance with an operational speed of the brakepedal. Generally, the operational speed of the brake pedal when anemergency braking is required is higher than that of the normal brakingoperation. Thus, according to the above-mentioned discriminating method,the operation requiring an emergency braking and the operation requiringa normal brake can be discriminated with high accuracy.

When an amount of depression of the brake pedal is detected, it isdetermined that the emergency braking is not required any more and thecontrol is switched from the brake assist control to the normal brakecontrol. In this case, a fluid pressure generated by the fluid pressuregenerating mechanism is reduced discontinuously due to switching shocks.Thus, a vibration is generated in the fluid pressure of the brake forcecontrol apparatus, and such vibration is transmitted to the mastercylinder or the brake pedal. If the vibration is transmitted to thebrake pedal, an operational speed of the brake pedal may be increasedand exceeds the above-mentioned predetermined value even though anamount of depression of the brake pedal is decreased. In this case, thebrake assist control is unnecessarily started. Additionally, a noise maybe superimposed on an output signal of the sensor which detects anoperational speed of the brake pedal. As mentioned above, in theabove-mentioned conventional apparatus, it is possible that an executionof the brake assist control is started immediately after the brakeassist control is terminated even though an emergency braking is notrequired.

DISCLOSURE OF INVENTION

It is an object of the present invention to provide an improved anduseful brake force control apparatus in which the above-mentionedproblems are eliminated.

A more specific object of the present invention is to provide a brakeforce control apparatus in which the brake assist control is notimproperly performed immediately after the brake assist control isterminated.

In order to achieve the above-mentioned objects, there is providedaccording to the present invention a brake force control apparatus of avehicle selectively performing a normal control for generating a brakeforce corresponding to a brake pressing force and a brake assist controlfor generating a brake force larger than that of the normal control,

characterized by:

control start determining means for determining whether or not the brakeassist control should be performed based on a change in a drivingcondition of the vehicle due to an operation of a brake pedal; and

control prohibiting means for prohibiting an execution of a subsequentbrake assist control operation after a previous brake assist controloperation was terminated and until a predetermined time has been passed.

In the present invention, the brake force control apparatus generates abrake force greater than that of a normal control time when the brakeassist control is being performed. Accordingly, when the brake assistcontrol is terminated, a sharp change is generated in a brake forcegenerated by the brake force control apparatus. In association with sucha change, a vibration is generated in a fluid pressure of the brakeforce control apparatus. Such a vibration influences an operationalstate of the brake pedal. Due to this change it is possible that a startcondition of the brake assist control is satisfied. In the presentinvention, an execution of the brake assist control is prohibited by thebrake assist control prohibiting means until an elapsed time after thebrake assist control was terminated reaches a predetermined time.Accordingly, the brake assist control is not performed if the conditionto start the brake assist control is satisfied immediately after thebrake assist control is terminated.

As mentioned above, according to the present invention, a start of thebrake assist control which should not be performed immediately after abrake assist control operation was terminated can be prevented byprohibiting a start of the brake assist control after the brake assistcontrol was terminated and until the predetermined time has been passed.

The control start determining means may determine whether or not thebrake assist control should be performed based on a pressure of a mastercylinder.

In one embodiment according to the present invention, the control startdetermining means prohibits the brake assist control when the pressureof the master cylinder is greater than a predetermined value.

Additionally, the control start determining means may prohibit the brakeassist control when a rate of change of the pressure of the mastercylinder is greater than a predetermined value.

Additionally, the predetermined time may preferably range from 100milliseconds to 200 milliseconds, and is more preferably 120milliseconds.

Additionally, the control start determining means determines whether ornot the brake assist control should be performed based on one of a brakepressing force, a pedal stroke of the brake pedal, a deceleration of thevehicle, a speed of the vehicle and a wheel speed of the vehicle

Other objects, features and advantages of the present invention willbecome more apparent from the following detailed descriptions when readin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a system structure diagram of a brake force control apparatusaccording to an embodiment of the present invention;

FIG. 2 is an illustration for showing a change in a brake pressing forceachieved under various circumstances;

FIG. 3 is a graph showing a change in a master cylinder pressure withrespect to time when a brake assist control is terminated;

FIG. 4 is a flowchart of an example of a control routine performed inthe brake force control apparatus shown in FIG. 1;

FIG. 5 is a system structure diagram of a brake force control apparatusaccording to a second embodiment of the present invention; and

FIG. 6 is a cross-sectional view of a vacuum booster used in the brakeforce control apparatus shown in FIG. 5.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a system structure diagram of a brake force control apparatusaccording to an embodiment of the present invention. The brake forcecontrol apparatus shown in FIG. 1 is controlled by an electronic controlunit 10 (hereinafter, referred to as ECU 10). The brake force controlapparatus comprises a pump 12. The pump 12 has a motor 14 as a powersource thereof. An inlet port 12a of the pump 12 communicates with areservoir tank 16. An accumulator 20 communicates with an outlet port12b of the pump via a check valve 18. The pump 12 delivers brake fluidin the reservoir tank 16 from the outlet port 12b so that apredetermined pressure is always accumulated in the accumulator 20.

The accumulator 20 communicates with a high-pressure port 24a of aregulator 24 via a high-pressure passage 22, and communicates with aregulator switching solenoid 26 (hereinafter, referred to as STR 26).The regulator 24 has a low-pressure port 24b and a control fluidpressure port 24c. The low-pressure port 24b communicates with thereservoir tank 16 via a low-pressure passage 28. The control fluidpressure port 24c communicates with the STR 26 via a control fluidpressure passage 29. The STR 26 is a two-position solenoid valve whichselectively set one of the control fluid pressure passage 29 and thehigh-pressure passage 22 in a conductive state, and sets the controlfluid pressure passage 29 in a conductive state and sets thehigh-pressure passage in a closed state in a normal state. Hereinafter,the two-position solenoid valve means a solenoid valve of which statecan be switched either one of two states.

A brake pedal 30 is connected to the regulator 24, and a master cylinderis mounted to the regulator 24. The regulator 24 has a fluid pressurechamber therein. The fluid pressure chamber always communicates with thecontrol fluid pressure port 24c, and selectively communicates with thehigh-pressure port 24a or the low-pressure port 24b in accordance withan operational state of the brake pedal 30. The regulator 24 isconfigured so that a pressure inside the fluid pressure chamber isadjusted to a fluid pressure corresponding to a brake pressing force FPexerted on the brake pedal 30. Accordingly, the fluid pressurecorresponding to the brake pressing force FP always appears at thecontrol fluid pressure port 24c of the regulator 24. Hereinafter, thisfluid pressure is referred to as a regulator pressure PRE.

The brake pressing force FP exerted on the brake pedal 30 ismechanically transmitted to a master cylinder 32 via the regulator 24.Additionally, a force corresponding to the fluid pressure inside thefluid pressure chamber of the regulator 24, that is, a forcecorresponding to the regulator pressure PRE, is transmitted to themaster cylinder 32.

The master cylinder 32 is provided with a first fluid pressure chamber32a and a second fluid pressure chamber 32b therein. A master cylinderpressure PM/C corresponding to a resultant force of the brake pressingforce FP and a brake assist force FA is generated in the first fluidpressure chamber 32a and the second fluid pressure chamber 32b. Both themaster cylinder pressure PM/C generated in the first fluid pressurechamber 32a and the master cylinder pressure PM/C generated in thesecond fluid pressure chamber 32b are supplied to a proportioning valve34 (hereinafter, referred to as P valve 34).

The P valve 34 communicates with a first fluid pressure passage 36 and asecond fluid pressure passage 38. The P valve 34 supplies the mastercylinder pressure PM/C to the first fluid pressure passage 36 and thesecond fluid pressure passage 38 without change in a range where themaster cylinder pressure PM/C is less than a predetermined value.Additionally, the P valve 34 supplies the master cylinder pressure PM/Cto the first fluid pressure passage 36 without change and supplies afluid pressure obtained by decreasing the master cylinder pressure PM/Cby a predetermined ratio to the second fluid pressure passage 38 in arange where the master cylinder pressure PM/C is less than apredetermined value.

A hydraulic pressure sensor 40, which outputs an electric signalcorresponding to the master cylinder pressure PM/C, is provided betweenthe second fluid pressure chamber 32b and the P valve 34. An outputsignal of the hydraulic pressure sensor 40 is supplied to the ECU 10.The ECU 10 detects the master cylinder pressure PM/C generated in themaster cylinder 32 based on the output signal of the hydraulic pressuresensor 40.

The above-mentioned STR 26 communicates with a third fluid pressurepassage 42. The third fluid pressure passage 42 communicates with one ofthe control fluid pressure passage 29 and the high-pressure passage 22in accordance with a state of the STR 26. In the present embodiment,wheel cylinders 44FL and 44FR provided to left and right front wheels FLand FR are provided with a brake fluid pressure from the first fluidpressure passage 36 communicating with the P valve 34 or the third fluidpressure passage 42 communicating with the STR 26. Additionally, wheelcylinders 44RL and 44RR provided to left and right rear wheels RL and RRare provided with a brake fluid pressure from the second fluid pressurepassage 38 communicating with the P valve 34 or the third fluid pressurepassage 42 communicating with the STR 26.

The first fluid pressure passage 36 communicates with a first assistsolenoid valve 46 (hereinafter referred to as SA-1 46) and a secondassist solenoid valve 48 (hereinafter, referred to as SA-2 48). On theother hand, the third fluid pressure passage 42 communicates with aright front holding solenoid valve 50 (hereinafter, referred to as SFRH50), a left front holding solenoid valve 52 (hereinafter, referred to asSFLH 52) and a third assist solenoid valve 54 (hereinafter, referred toas SA-3 54).

The SFRH 50 is a two-position solenoid valve which maintains an openstate in a normal state. The SFRH 50 communicates with the SA-1 46 and aright front wheel pressure decreasing solenoid valve 58 (hereinafter,referred to as SFRR 58) via a pressure adjusting fluid pressure passage56. A check valve 60 permitting a fluid flow only in a direction fromthe pressure adjusting fluid pressure passage 56 to the third fluidpressure passage 42 is provided, in parallel, between the third fluidpressure passage 42 and the pressure adjusting fluid pressure passage56.

The SA-1 46 is a two-position solenoid valve which selectively rendersone of the first fluid pressure passage 36 and the pressure adjustingfluid pressure passage 56 to communicate with the wheel cylinder 44FR,and renders the first fluid pressure passage 36 and the wheel cylinder44FR to be in a communicating state in a normal state (OFF state). Onthe other hand, the SFRR 58 is a two-position solenoid valve whichrenders the pressure adjusting fluid pressure passage 56 and thereservoir tank 16 to be in a connected state or a disconnected state.The SFRR 58 renders the pressure adjusting fluid pressure passage 56 andthe reservoir tank 16 to be in a disconnected state in a normal state(OFF state).

The SFLH 52 is a two-position solenoid valve which maintains an openstate in a normal state. The SFLH 52 communicates with the SA-2 48 and aleft front wheel pressure decreasing solenoid valve 64 (hereinafter,referred to as SFLR 64) via a pressure adjusting fluid pressure passage62. A check valve 66 permitting a fluid flow only in a direction fromthe pressure adjusting fluid pressure passage 62 to the third fluidpressure passage 42 is provided, in parallel, between the third fluidpressure passage 42 and the pressure adjusting fluid pressure passage62.

The SA-2 48 is a two-position solenoid valve which selectively rendersone of the first fluid pressure passage 36 and the pressure adjustingfluid pressure passage 62 to communicate with the wheel cylinder 44FL,and renders the first fluid pressure passage 36 and the wheel cylinder44FL to be in a communicating state in a normal state (OFF state). Onthe other hand, the SFLR 64 is a two-position solenoid valve whichrenders the pressure adjusting fluid pressure passage 62 and thereservoir tank 16 to be in a connected state or a disconnected state.The SFLR 64 renders the pressure adjusting fluid pressure passage 62 andthe reservoir tank 16 to be in a disconnected state from each other in anormal state (OFF state).

The second fluid pressure passage 38 communicates with theabove-mentioned SA-3 54. The downstream side of the SA-3 54 communicateswith a right rear wheel holding solenoid valve 68 (hereinafter, referredto as SRRH 68) provided in correspondence with a wheel cylinder 44RR ofthe right rear wheel RR and a left rear wheel holding solenoid valve 70(hereinafter, referred to as SRLR 70) provided in correspondence with awheel cylinder 44RL of the left rear wheel RL. The SA-3 54 is atwo-position solenoid valve which selectively selectively renders one ofthe second fluid pressure passage 38 and the third fluid pressurepassage 42 to communicate with the SRRH 68 and the SRLR 70, and rendersthe second fluid pressure passage 38, the SRRH 68 and the SRLR 70 in acommunicating state in a normal state (OFF state).

The downstream side of the SRRH 68 communicates with the wheel cylinder44RR and a right rear wheel pressure decreasing solenoid valve 74(hereinafter, referred to as SRRR 74) via a pressure adjusting fluidpressure passage 72. The SRRR 74 is a two-position solenoid valve whichrenders the pressure adjusting fluid pressure passage 72 and thereservoir tank 16 in a communicating state or a disconnected state, andrenders the pressure adjusting fluid pressure passage 72 and thereservoir tank 16 in the disconnected state in a normal state (OFFstate). Additionally, a check valve 76 permitting a fluid flow only in adirection from the pressure adjusting fluid pressure passage 72 to theSA-3 54 is provided, in parallel, between the SA-3 54 and the pressureadjusting fluid pressure passage 72.

Similarly, the downstream side of the SRLH 70 communicates with thewheel cylinder 44RL and a left rear wheel pressure decreasing solenoidvalve 80 (hereinafter, referred to as SRLR 80) via a pressure adjustingfluid pressure passage 78. The SRLR 80 is a two-position solenoid valvewhich renders the pressure adjusting fluid pressure passage 78 and thereservoir tank 16 in a communicating state or a disconnected state, andrenders the pressure adjusting fluid pressure passage 78 and thereservoir tank 16 in the disconnected state in a normal state (OFFstate). Additionally, a check valve 82 permitting a fluid flow only in adirection from the pressure adjusting fluid pressure passage 78 to theSA-3 54 is provided, in parallel, between the SA-3 54 and the pressureadjusting fluid pressure passage 78.

In the system according to the present embodiment, a brake switch 84 isprovided near the brake pedal 30. The brake switch 84 is a switch thatgenerates an ON output when the brake pedal 30 is pressed. The outputsignal of the brake switch 84 is supplied to the ECU 10. The ECU 10determines whether or not a braking operation is performed by the driverbased on the output signal of the brake switch 84.

Additionally, in the system according to the present embodiment, wheelspeed sensors 86FL, 86FR, 86RL and 86RR (hereinafter, these are referredto as 86** as a whole) are provided near the left and right front wheelsFL and FR and the left and right rear wheels RL and RR, each of thesensors generating a pulse signal when the respective wheel rotates apredetermined angle. The output signals of the wheel speed sensors 86**are supplied to the ECU 10. The ECU 10 detects a wheel speed of each ofthe wheels FL, FR, RL and RR based on the output signals of the wheelspeed sensors 86**.

The ECU 10 supplies, if necessary, drive signals to the above-mentionedSTR 26, SA-1 46, SA-2 4E3, SA-3 54, SFRH 50, SFLH 52, SFRR 58, SFLR 64,SRRH 68, SRLH 70, SRRR 74 and SRLR 80 based on the output signal of thebrake switch 84.

A description will now be given of an operation of the brake forcecontrol apparatus according to the present embodiment. The brake forcecontrol apparatus according to the present embodiment performs thenormal control for generating a brake force corresponding to the brakepressing force FP exerted on the brake pedal 30 when the vehicle is in astable state. The normal control can be achieved, as shown in FIG. 1, byturning off all of the STR 26, SA-1 46, SA-2 48, SA-3 54, SFRH 50, SFLH52, SFRR 58, SFLR 64, SRRH 68, SRLH 70, SRRR 74 and SRLR 80 based on theoutput signal of the brake switch 84.

That is, in the state shown in FIG. 1, the wheel cylinders 44FR and 44FLcommunicate with the first fluid pressure passage 36, and the wheelcylinders 44RR and 44RL communicate with the second fluid pressurepassage 38. In this case, the brake fluid flows between the mastercylinder 32 and the wheel cylinders 44FR, 44FL, 44RL and 44RR(hereinafter, these may be referred to as 44** as a whole), and a brakeforce corresponding to the brake pressing force FP is generated in eachof the wheels FL, FR, RL and RR.

In the present embodiment, when a possibility for shifting to a lockedstate is detected in one of the wheels, it is determined that acondition for performing an antilock brake control (hereinafter,referred to as ABS control) is established and the ABS control isstarted thereafter. The ECU 10 calculates wheel speeds VWFL, VWFR, VWRLand VWRR (hereinafter, these are referred to as VW** as a whole) of thewheels based on output signals of the wheel speed sensors 86**.Additionally, the ECU 10 calculates an assumed value VSO (hereinafter,referred to as an assumed vehicle speed VSO) of a speed of the vehicleaccording to a publicly known method. Then, when the vehicle is in abraking state, the ECU 10 calculates a slip rate S of each wheelaccording to the following equation so as to determine that the wheelmay shift to a locked state when the slip rate S exceeds a predeterminedvalue.

    S=(VSO-VW**)·100/VSO                              (1)

When the condition for performing the ABS control is established, theECU 10 outputs the drive signals to the SA-1 46, SA-2 48 and SA-3 54. Asa result, when the SA-1 46 is turned on, the wheel cylinder 44FR isdisconnected from the first fluid pressure passage 36 and connected tothe pressure adjusting fluid pressure passage 56. Additionally, when theSA-2 48 is turned on, the wheel cylinder 44FL is disconnected from thefirst fluid pressure passage 36 and connected to the pressure adjustingfluid pressure passage 62. Further, when the SA-3 54 is turned on, theupstream side of the SRRH 68 and the SRLH 70 is disconnected from thesecond fluid pressure passage 38 and connected to the third fluidpressure passage 42.

In this case, all wheel cylinders 44** communicate with respectiveholding solenoid valves SFRH 50, SFLH 52, SRRH 68 and SRLH 70(hereinafter, these are referred to as holding solenoid S**H) andrespective pressure decreasing solenoid valves SFRR 58, SFLR 64, SRRR 74and SRLR 80 (hereinafter, these are referred to as pressure decreasingsolenoid S**R), and a regulator pressure PRE is introduced to theupstream side of each of the holding solenoids S**H via the third fluidpressure passage 42 and the STR 26.

In the above-mentioned condition, a wheel cylinder pressure PW/C of therespective wheel cylinders 44** is increased with the regulator pressurePRE as an upper limit by the holding solenoids S**H being in an openstate and the pressure decreasing solenoids S**R being in a closedstate. Hereinafter, this state is referred to as a pressure increasingmode 1. Additionally, the wheel cylinder pressure PW/C of the respectivewheel cylinders 44** is maintained without being increased or decreasedby the holding solenoids S**H being in a closed state and the pressuredecreasing solenoids S**R being in the closed state. Hereinafter, thisstate is referred to as a holding mode 2. Further, the wheel cylinderpressure PW/C of the respective wheel cylinders 44** is decreased by theholding solenoids S**H being in the closed state and thepressure-decreasing solenoids S**R being in the open state. Hereinafter,this state is referred to as a pressure decreasing mode 3. The ECU 10sets, if necessary, the above-mentioned pressure increasing mode 1,holding mode 2 and pressure decreasing mode 3 so that a slip rate S ofeach wheel during a braking time becomes an appropriate value, that is,so that each wheel does not shift to the locked state.

When a depression of the brake pedal 30 is released by the driver duringexecution of the ABS control, the wheel cylinder pressure PW/C must beimmediately decreased. In the system according to the presentembodiment, the check valves 60, 66, 76 and 82 are provided in ahydraulic pressure paths corresponding to each of the wheel cylinders44**, each of the check valves 60, 66, 76 and 82 permitting a fluid flowonly in the directions from the wheel cylinders 44** to the third fluidpressure passage 42. Thus, according to the system of the presentembodiment, the wheel cylinder pressures PW/C of all of the wheelcylinders 44** can be immediately decreased after the depression of thebrake pedal 30 is released.

In the system according to the present embodiment, when the ABS controlis performed, the wheel cylinder pressure PW/C is increased by the brakefluid being supplied from the regulator 24 to the wheel cylinders 44**,that is, by the brake fluid being supplied from the pump 12 to the wheelcylinders 44**, and is decreased by the brake fluid in the wheelcylinders 44** flowing to the reservoir tank 16. When the increase inthe wheel cylinder pressure PW/C is performed by using the mastercylinder 32 as a fluid pressure source and if the pressure increasingmode and the pressure decreasing mode are repeatedly performed, thebrake fluid in the master cylinder 32 gradually decreases and aso-called bottoming of the master cylinder may occur.

On the other hand, if the pump 12 is used as a fluid pressure source soas to increase the wheel cylinder pressure PW/C, as in the systemaccording to the present embodiment, such a bottoming can be prevented.Thus, in the system according to the present embodiment, a stableoperational state can be maintained if the ABS control is continued fora long time.

In the system according to the present embodiment, the ABS control isstarted when a possibility for shifting to the locked state is detectedin one of the wheels. Accordingly, in order to start the ABC; control,as a precondition, a braking operation having a level at which a largeslip rate S is generated in one of the wheels must be performed.

FIG. 2 shows changes in the brake pressing force FP applied to the brakepedal 30 with respect to time under various conditions. Curves indicatedby 1 and 2 in FIG. 2 represent changes in the pressing force FP when anemergency braking is performed by a highly skilled driver (hereinafter,referred to as a high-grade driver) and an unskilled driver or a driverlacking strength (hereinafter, referred to as a beginner-grade driver),respectively. The emergency braking operation is an operation performedwhen is it desired to rapidly decelerate a vehicle. Accordingly, thebrake pressing force associated with the emergency braking operation ispreferably a force sufficiently large as the ABS control is performed.

As shown by the curve 1, when the driver of the vehicle is a high-gradedriver, the brake pressing force FP is immediately and rapidly increasedin response to establishment of a condition in which an emergencybraking is required, and a large brake pressing force FP can bemaintained for a long time. If such a brake pressing force FP is exertedon the brake pedal 30, a sufficiently high brake fluid pressure can beprovided from the master cylinder 32 to each of the wheel cylinders 44**so as to start the ABS control.

However, as shown by the curve 2 when the driver of the vehicle is abeginner-grade driver, the brake pressing force FP may not be increasedto a sufficiently high value in response to establishment of thecondition in which an emergency braking is required. If the brakepressing force FP exerted on the brake pedal 30 is not sufficientlyincreased as shown by the curve 2 after an emergency braking isrequired, the wheel cylinder pressure PW/C in each of the wheels 44** isnot sufficiently increased, which results in a possibility that the ABScontrol is not started.

As mentioned above, when the driver of the vehicle is a beginner-gradedriver, the braking ability of the vehicle may not be sufficientlyperformed even when an emergency braking operation is performed despitethat the vehicle has a good braking ability. Accordingly, the systemaccording to the present embodiment is provided with a brake assistfunction for sufficiently increasing the wheel cylinder pressure PW/Ceven if the brake pressing force FP is not sufficiently increased whenthe brake pedal is operated with an intention to perform an emergencybraking Hereinafter, a control performed by the ECU 10 to achieve such afunction is referred to as a brake assist control.

In the system according to the present embodiment, when performing thebrake assist control, an accurate determination must be made as towhether, when the brake pedal 30 is operated, the operation is intendedto perform an emergency braking operation or to perform a regularbraking operation.

Curves indicated by shown 3 and 4 in FIG. 2 show changes in the brakepressing force FP when the driver operates the brake pedal with anintention to perform a normal braking operation under variousconditions. As shown by the curves 1 to 4, a change in the brakepressing force FP associated with the normal braking operation is gentleas compared to a change in the brake pressing force FP associated withan emergency braking operation. Additionally, a convergent value of thebrake pressing force FP associated with the normal braking operation isnot so large as a convergent value of the brake pressing force FPassociated with an emergency braking operation.

Giving attention to those differences, when the brake pressing force FPis increased to a sufficiently large value at a rate of change exceedinga predetermined value after a braking operation is started, that is,when the brake pedal 30 is operated so that the brake pressing force FPreaches an area indicated by (I) in FIG. 2, it can be determined that anemergency braking is performed.

Additionally, when the rate of change of the brake pressing force FP issmaller than the predetermined value or when the convergent value of thebrake pressing force FP is smaller than the predetermined value, thatis, when the brake pedal 30 is operated so that the brake pressing forceFP always changes within an area indicated by (II) in FIG. 2, it can bedetermined that a normal braking operation is performed.

Accordingly, in the system according to the present embodiment, anoperational speed and an amount of operation of the brake pedal isdetected or assumed, and, then, it is determined whether or not theoperational speed exceeds the predetermined value and whether or not theamount of operation exceeds the predetermined value, and, thereby, itcan be determined whether or not the operation to the brake pedal 30 isintended to perform an emergency braking.

In a vehicle provided with the brake force control apparatus accordingto the present embodiment, the brake pedal 30 is moved by an increase ordecrease in the brake pressing force FP. At this time, a largeroperational speed is generated in the brake pedal 30 as the brakepressing force shows a steep slope, and an amount of operationsubstantially corresponding to the brake pressing force FP is generated.Accordingly, the operational speed and the amount of operation of thebrake pedal 30 can be accurately assumed from the brake pressing forceFP.

When the brake pressing force FP is exerted on the brake pedal 30, astroke L corresponding to the brake pressing force FP is generated inthe brake pedal 30. Additionally, when the stroke L is generated in thebrake pedal 30, a master cylinder pressure PM/C corresponding to thestroke L, which corresponds to the brake pressing force FP, is generatedin the master cylinder 32. When the master cylinder pressure PM/Ccorresponding to the brake pressing force FP is generated, a vehicledeceleration G corresponding to the brake pressing force FP is generatedin the vehicle. Accordingly, en operational speed and an amount ofoperation of the brake pedal 30 can be assumed from parameters including2 the pedal stroke L, 3 the master cylinder pressure PM/C, 4 the vehicledeceleration G, 5 the assumed vehicle speed VSO and 6 the wheel speedVW**, other than the above-mentioned 1 brake pressing force FP.

In order to accurately assume an operational speed and an amount ofoperation of the brake pedal 30, that is, in order to accuratelydiscriminate an emergency braking and a normal brake, preferredparameters of the above-mentioned parameters (hereinafter, referred toas basic parameters) are those detected at positions closest to the footof the driver. According to such a pint of view, the parameters 1 to 6have a superiority in the order of 1→6 when used as the basicparameters.

In order to detect 1 the brake pressing force FP, it is required toprovide (i) a pressing force sensor. Additionally, in order to detect 2the pedal stroke L, it is required to provide (ii) a stroke sensor.Similarly, in order to detect 3 the master cylinder pressure PM/C and 4the vehicle deceleration G, it is required to provide a (iii) ahydraulic pressure sensor and (iv) a deceleration sensor, respectively.Further, in order to detect 5 the assumed vehicle speed VSO and 6 thewheel speed VW**, it is required to provide (v) a wheel speed sensor.

The (v) wheel speed sensor and the (iv) deceleration sensor among theabove-mentioned sensors (i) to (v) are conventionally and widely usedsensors for a vehicle. On the other hand, the (ii) stroke sensor and the(i) pressing force sensor are not popular sensors for a vehicle.Accordingly, considering a cost merit of a sensor due to a massproduction effect, the above-mentioned sensors (i) to (v) have asuperiority in the order of (v)→(i).

In the system according to the present embodiment, considering theabove-mentioned merit and demerit, the hydraulic pressure sensor 40 isused as a sensor for detecting the basic parameters so as todiscriminate an emergency braking operation and a normal brakingoperation by using the master cylinder pressure PM/C as a basicparameter. A description will now be given of an operation of the systemaccording to the present embodiment when it is determined by the ECU 10that an emergency braking is performed.

The ECU 10 determines that an emergency braking is performed when themaster cylinder pressure PM/C exceeding the predetermined value isdetected and a rate of change ΔPM/C is detected after the brake pedal 30is pressed. When it is determined that an emergency braking isperformed, the ECU 10 outputs the drive signals to the STR 26, the SA-146, the SA-2 48 and the SA-3 54.

When the STR 26 is turned on upon receipt of the above-mentioned drivesignal, the third fluid pressure passage 42 and the high-pressurepassage 22 are directly connected to each other. In this case, anaccumulator pressure PACC is introduced into the third fluid pressurepassage 42. Additionally, when the SA-1 46 and the SA-2 48 are turned onupon receipt of the drive signals, the wheel cylinders 44FR and 44FLcommunicate with the pressure adjusting fluid pressure passages 56 and62, respectively. Further, when the SA-3 54 is turned on upon receipt ofthe above-mentioned drive signal, the upstream side of the SRRH 68communicates with the third fluid pressure passage 42. In this case, astate is established in which all of the wheel cylinders 44**communicate with the respective holding solenoids S**H and therespective pressure decreasing solenoids S**R and the accumulatorpressure PACC is introduced to the upstream side of each of the holdingsolenoids S**H.

In the ECU 10, all of the holding solenoids S**H and all of the pressuredecreasing solenoids S**R are maintained in the OFF state immediatelyafter execution of an emergency braking is detected. Accordingly, asmentioned above, when the accumulator pressure PACC is introduced to theupstream side of the holding solenoids S**H, the fluid pressure isprovided to the wheel cylinders 44** without being changed. As a result,the wheel cylinder pressure PW/C of all of the wheel cylinders 44** isincreased toward the accumulator pressure PACC.

As mentioned above, according to the system of the present embodiment,when an emergency braking is performed, the wheel cylinder pressure PW/Cof all of the wheel cylinders 44** can be immediately increasedirrespective of a magnitude of the brake pressing force FP. Thus,according to the system of the present embodiment, a large brake forcecan be generated immediately after establishment of a condition in whichan emergency braking is required, even if the driver is a beginner-gradedriver.

When the accumulator pressure PACC begins to be supplied to the wheelcylinders 44**, as mentioned above, a slip rate S of each of the wheelsFL, FR, RL and RR is rapidly increased, and the condition for performingthe ABS control is finally established. When the condition forperforming the ABS control is established, the ECU 10 set, if necessary,the above-mentioned pressure-increasing mode 1, holding mode 2 andpressure-decreasing mode 3 so that the slip rate S of each of the wheelbecomes an appropriate value, that is, so that each of the wheels doesnot shift to the locked state.

It should be noted that when the ABS control is performed subsequent toan emergency braking operation, the wheel cylinder pressure PW/C isincreased by using the pump 12 and the accumulator 20 as a fluidpressure source, and is decreased by the brake fluid in the wheelcylinders 44** flowing to the reservoir tank 16. Accordingly, if thepressure increasing mode and the pressure decreasing mode are repeated,a so-called bottoming of the master cylinder 32 does not occur.

When the brake assist control is started as mentioned above by executionof an emergency braking operation, the brake assist control must beended when a press of the brake pedal 30 is released. In the systemaccording to the present invention, as mentioned above, the STR 26, theSA-1 46, the SA-2 48 and the SA-3 54 are maintained to be in the ONstate. When the STR 26, the SA-1 46, the SA-2 48 and the SA-3 54 are inthe ON state, each of the fluid pressure chamber in the regulator 24 andthe first fluid pressure chamber 32a and the second fluid pressurechamber 32b becomes substantially a closed space.

Under the above-mentioned condition, the master cylinder pressure PM/Cbecomes a value corresponding to the brake pressing force FP.Accordingly, by monitoring the output signal of the master cylinderpressure PM/C detected by the hydraulic pressure sensor 40, it can beeasily determined whether or not a depression of the brake pedal 30 ishas been released. When the release of the press of the brake pedal 30is detected, the ECU 10 stops the supply of the drive signals to the STR26, the SA-1 46, the SA-2 48 and the SA-3 54 so as to perform the normalcontrol.

As for the basic parameters which are the basis of discriminationbetween an emergency braking and a normal brake, 1 the brake pressingforce FP, 2 the pedal stroke L, 4 the vehicle deceleration G, 5 theassumed vehicle speed VSO and 6 the wheel speed VW** other than theabove-mentioned 3 master cylinder pressure PM/C may be applicable. Amongthose parameters, the 1 brake pressing force FP and 2 the pedal stroke Lare parameters that are sensitive to a change in the brake pressingforce FP, similar to 3 the master cylinder pressure PM/C. Accordingly,when 1 the brake pressing force FP or 2 the pedal stroke L is used as abasic parameter, it can be easily determined whether or not the press ofthe brake pedal 30 is released by monitoring the parameter.

On the other hand, the parameters 4 to 6 vary when a brake force of eachwheel is changed. If the depression of the brake pedal 30 is released,there is no large changes generated in these parameters. Accordingly,when the parameters 4 to 6 are used as the basic parameter, it iseffective to perform a determination for a termination of the brakeassist control based on the output state of a pressing force switch thatis provided for detecting whether or not the brake pressing force FP isapplied.

As mentioned above, when the brake assist control is terminated, the STR26, the SA-1 46, the SA-2 48 and the SA-3 54 changed from the ON stateto the OFF state. In association with such a change, as mentioned abovethe first and second fluid pressure chambers 32a and 32b of the mastercylinder are changed from a state in which they are caused to be aclosed space to a state in which they communicate with the wheelcylinder 44FL**. When the brake assist control is being performed, ahigh pressure of the accumulator 20 is provided to the wheel cylinder44FL**. On the other hand, the brake assist control is terminated at atime when a depression of the brake pedal is decreased to less than apredetermined degree. Accordingly, a pressure in each chamber of themaster cylinder 32 immediately before the brake assist control isterminated is decreased to a value which is sufficiently lower than thewheel cylinder pressure. Accordingly, when the first and second fluidpressure chambers 32a and 32b are changed to the state in which theycommunicate with the wheel cylinder 44FL** as mentioned above, a brakefluid in the wheel cylinder 44FL** rapidly flows into the mastercylinder 32. Such a rapid movement of the brake fluid generates a wavingmotion in the brake fluid, which results in a vibration generated in themaster cylinder pressure PM/C.

FIG. 3 shows a change in the master cylinder pressure PM/C with respectto time when an execution of the brake assist control is terminated. Asshown in FIG. 3, a vibration is generated at a time when the brake(assist control is terminated. Due to the vibration, PM/C and ΔPM/C aretemporarily increased, and it is possible to be determined that anemergency braking is required. As mentioned above, if a determination ismade based only the fact as to whether or not the master cylinderpressure PM/C and the rate of change ΔPM/C thereof exceeds thepredetermined value, it is possible that the brake assist control isrestarted immediately after the brake assist control is terminated eventhough an emergency braking is not required any more. In such a case,since the above-mentioned vibration rapidly attenuates, an end conditionis immediately established and the brake assist control is terminated.Thereafter, on and off of the brake assist control is vibratoryrepeated, resulting in an incongruous feel to passengers of the vehicle.

The brake force control apparatus according to the present embodimenthas a feature that an improper execution of the brake assist controlcaused by the vibration in the master cylinder pressure PM/C associatedwith the change in the control state by prohibiting a start of the brakeassist control after the brake assist control was terminated and untilthe predetermined time has been passed.

A description will now be given, with reference to FIG. 4, of contentsof a process performed by the ECU 10 so as to achieve theabove-mentioned function. FIG. 4 is a flowchart of an example of acontrol routine performed by the ECU 10. It should be noted that theroutine shown in FIG. 4 is a periodic interruption routine which isstarted at every predetermined time. When the routine shown in FIG. 4 isstarted, the process of step 100 is performed first.

In step 100, it is determined whether or not the master cylinderpressure PM/C is larger than a predetermined value α. The predeterminedvalue α is a value which is not output when the hydraulic pressuresensor 40 is normally operated. Accordingly, if it is determined thatPM/C>α is established, it can be determined that an abnormality occursin the hydraulic pressure sensor 40. In this case, the process of step102 is performed subsequently. On the other hand, if it is determinedthat PM/C>α a is not established, the process of step 104 is performed.

In step 102, execution of the brake assist control is prohibited.Accordingly, when an abnormality occurs in the hydraulic pressure sensor40, the control is not continued based on an abnormal master cylinderpressure PM/C. After the process of step 102 is completed, the routineat this time is ended.

In step 104, it is determined whether or not the rate of change ΔPM/C ofthe master cylinder pressure PM/C is greater than a predetermined valueβ. The predetermined value β is a value which is not generated when thehydraulic pressure sensor 40 normally outputs the master cylinderpressure PM/C. Accordingly, if it is determined that ΔPM/C>β isestablished, it can be determined that a noise is superimposed on theoutput signal of the hydraulic pressure sensor 40. In this case, theprocess of step 102 is performed subsequently. Thus, according to thebrake force control apparatus of the present embodiment, an impropercontrol is not performed due to an influence of a noise. On the otherhand, if it is determined that ΔPM/C>β is not established, the processof step 106 is performed next.

In step 106, it is determined whether or not a passe time T after theprevious brake assist control was terminated is smaller than apredetermined time T0. As a result, if it is determined that T<T0 isstill established, the process of step 102 is performed subsequently. Onthe other hand, if it is determined that T<T0 is not established, theprocess of step 108 is performed subsequently.

It should be noted that the predetermined time T0 is set to be greaterthan a time T1 which is necessary for the vibration to be sufficientlyconverged when the brake assist control is terminated and smaller than ashortest time T2 which a time after the brake assist control wasterminated and until the brake pedal is pressed to a degree such that anemergency braking is required. In the brake force control apparatus, thevibration generated when the brake assist control is terminatedattenuates within about 100 ms. Additionally, when the driver repeats adepression and release of the brake pedal, a period of such a repetitionis about 200 ms at the shortest even if the driver acts very quickly.That is, the shortest time T2 after the brake assist control wasterminated and until the brake pedal is pressed to a degree such that anemergency braking is required is less than 200 ms. Accordingly, thepredetermined time to may be set within a range from 100 to 200 ms. Inthe present embodiment, T0 is se to 120 ms, for example.

In step 108, it is determined whether or not the start condition of thebrake assist control is established. A determination as to whether ornot the start condition of the bake assist control is established can bedetermined whether or not each of the master cylinder pressure PM/C andthe rate of change ΔPM/C thereof exceeds the predetermined value.

If it is determined, in step 108, that the start condition of the brakeassist control is established, it is determined that an emergencybraking operation is established and the process of step 110 isperformed subsequently. On the other hand, if it is determined that theabove mentioned start condition is not established, the routine at thistime is ended without performing any process thereafter.

In step 110, an execution of the brake assist control is started.Thereafter, a degree of depression of the brake pedal is reduced, andthe brake assist control is continued until the master cylinder pressurePM/C is decreased by more than a predetermined amount. After the processof step 110 is completed, the process at this time is ended.

According to the above-mentioned control routine, if the elapsed timeafter the brake assist control is terminated is less than thepredetermined time T0, an execution of the brake assist control isprohibited. Thereby, if the start condition of the brake assist controlis established due to the vibration of the master cylinder pressure PM/Cimmediately after the brake assist control is terminated, the brakeassist control is prevented from being restarted improperly. In thiscase, as mentioned above, it is enabled to prevent the improper restartof the brake assist control by setting the predetermined time to t0 avalue within a range from T1 to T2 while the brake assist control isalways performed when an emergency control is required.

It should be noted that, as mentioned above, 1 the brake pressing forceFP, 2 the pedal stroke L, 4 the vehicle deceleration G, 5 the assumedvehicle speed VSO and 6 the wheel speed VW** in addition to theabove-mentioned 3 master cylinder pressure PM/C may be applicable asbasic parameters. The vibration generated in the master cylinderpressure PM/C is transmitted to the brake pedal. Accordingly, avibration is generated in 2 the pedal stroke L. Additionally, at thetime when the brake assist control is terminated, the pressing forceapplied to the brake pedal is reduced but continuously applied. Thereby,when the vibration is transmitted to the brake pedal, a vibration isgenerated in 1 the brake pressing force FP irrespective of an intentionof the driver. Further, when the brake assist control is terminated, avibratory component appears in the behavior of the vehicle since a brakeforce is rapidly reduced. Thus, a vibration is generated also in 4 thevehicle deceleration G, 5 the assumed vehicle speed VSO and 6 the wheelspeed VW**. As mentioned above, since a vibration is generated in thebasic parameters when the brake assist control is terminated even if anyone of parameters 1 to 6 is used as the basic parameter, the startcondition for the brake assist control may be established. Accordingly,the above-mentioned control routine can be effectively applied when theparameters other than 3 the master cylinder pressure PM/C is used as thebasic parameter.

A description will now be given, with reference to FIG. 5 and FIG. 6, ofa second embodiment according to the present invention. FIG. 5 shows asystem structure diagram of a brake force control apparatus according tothe present invention. It should be noted that, in FIG. 5, only a partof the brake force control apparatus corresponding to one wheel is shownfor the sake of convenience.

The brake force control apparatus shown in FIG. 5 is controlled by anECU 200. The brake forces control apparatus according to the presentembodiment has a brake pedal 202. A brake switch 203 is provided nearthe brake pedal 202. The brake switch 203 is a switch which generates anON output when the brake pedal 202 is pressed. The output signal of thebrake switch 203 is supplied to the ECU 200. The ECU 200 determineswhether or not a braking operation is being performed based on theoutput signal of the brake switch 203.

The brake pedal 202 is connected to a vacuum booster 204. The vacuumbooster 204 is an apparatus which assists a brake pressing force byusing an intake negative pressure of an internal combustion engine as apower source. The brake force control apparatus according to the presentembodiment has a feature to generate an assist power having apredetermined power ratio with respect to a brake pressing force FP whena normal braking operation is performed, and generate a maximum assistpower irrespective of the brake pressing force FP when an emergencybraking is performed. A structure of the vacuum booster 204 will bedescribed later.

A master cylinder 206 is fixed to the vacuum booster 204. The mastercylinder 206 has a fluid pressure chamber therein. Additionally, areservoir tank 208 is provided above the master cylinder 206. The fluidpressure chamber of the master cylinder and the reservoir tank 208communicate with each other when a press of the brake pedal 202 isreleased, whereas they are disconnected from each other when the brakepedal is pressed. Accordingly, brake fluid is supplied to the fluidpressure chamber each time the press of the brake pedal 202 is released.

The fluid pressure chamber of the maser cylinder 206 communicates with afluid pressure passage 210. The fluid pressure passage 210 is providedwith a hydraulic pressure sensor 212 which outputs an electric signalcorresponding to a pressure inside the fluid pressure passage 210. Theoutput signal of the hydraulic pressure sensor 212 is supplied to theECU 200. The ECU 200 detects a fluid pressure generated by the mastercylinder 206, that is, the master cylinder pressure PM/C based on theoutput signal of the hydraulic pressure sensor 212.

The fluid pressure passage 210 is provided with a holding solenoid 216(hereinafter, referred to as SH 216). The SH 216 is a two-positionsolenoid valve which maintains an open state in a normal state (OFFstate). The SH 216 is set to be in an ON state (closed state) by a drivesignal being supplied by the ECU 200.

The downstream side of the SH 216 communicates with a wheel cylinder 218and a pressure decreasing solenoid 220 (hereinafter, referred to asSR220). The SR 220 is a two-position solenoid valve which maintains aclosed state in a normal state (OFF state). SR 220 is set to be in an ONstate (open state) by a drive signal being supplied by the ECU 200.Additionally, a checks valve 222 which permits a fluid flow only in adirection from the wheel cylinder 218 to the fluid pressure passage 210is provided between the wheel cylinder 218 and the fluid pressurepassage 210.

It should be noted that a wheel speed sensor 219 which generates a pulsesignal each time the wheel rotates a predetermined angle is providednear the wheel cylinder 218. An output signal of the wheel speed sensor219 is supplied to the ECU 200. The ECU 200 detects a wheel speed basedon the output signal of the wheel speed sensor 219.

A reservoir 224 is provided on the downstream side of the SR 220. Thebrake fluid flowing out of the SR 220 when the SR 220 is set to be inthe ON state (open state) is stored in the reservoir 224. It should benoted that the reservoir previously stores a predetermined amount ofbrake fluid. The reservoir 224 communicates with an inlet port 226a of apump 226. Additionally, an outlet port 226b of the pump 226 communicateswith the fluid pressure passage 210 via a check valve 228. The checkvale 228 is a one-way valve which permits a fluid flow only in adirection from the pump 226 to the fluid pressure passage 210.

A description will now be given of a structure of the vacuum booster 204and a structure of a periphery thereof. FIG. 6 shows a structure of thevacuum booster 204 and a structure of a periphery thereof. It should benoted that, in FIG. 6, the master cylinder 206 (not shown in FIG. 6 isfixed to the vacuum booster 204 on the left side thereof. Additionally,the brake pedal 202 (not shown in FIG. 6) is connected to the vacuumbooster 204 on the right side thereof.

The vacuum booster 204 includes a housing 234 which comprises a frontshell (F/S) 230 and a rear shell (RIS) 232. A diaphragm 236 and acylinder member 238 are provided inside the housing 232. The cylindermember 238 is a cylindrical, elastic member having a side surface formedin bellows so that the cylinder member can be elongated and compressedin leftward and rightward directions in FIG. 6. An inner space of thehousing 234 is divided into a negative pressure camber 240, a firstpressure changing chamber 242 and a second pressure changing chamber 244by the diaphragm 236 and the cylinder member 238.

The front shell 230 is provided with a negative pressure introducingport 246 which communicates with the negative pressure chamber 240. Thenegative pressure introducing port 246 communicates with a negativepressure passage 248 which communicates with a negative pressure sourcesuch as, for example, an intake passage of an internal combustionengine. The front shell 230 is also provided with a adjusting pressureintroducing port 250 which communicates with the second pressurechanging chamber 244. The adjusting pressure introducing port 250communicates with a negative pressure introducing valve 252 and anadjusting pressure passage 256 which also communicates with anatmospheric pressure introducing valve 254.

The negative pressure introducing valve 252 is a two-position solenoidvalve which is positioned between the adjusting pressure passage 256 andthe negative pressure passage 248, and maintains an open state in anormal state (OFF state). On the other hand, the atmospheric pressureintroducing valve 254 is a two-position solenoid valve which controlscommunication between the adjusting pressure passage 256 and anatmosphere, and maintains a closed state in a normal state (OFF state).The negative pressure introducing valve 252 and the atmospheric pressureintroducing valve 254 are rendered to be in the ON state (closed stateor open state, respectively), by a drive signal being supplied by theECU 200.

The rear shell 232 is provided with cm atmospheric pressure introducingport 258 which communicates with the first pressure changing chamber242. The atmospheric pressure introducing port 258 communicates with theadjusting pressure passage 256 via a check valve 260. The check valve260 is a one-way valve which permits a fluid flow only in a directionfrom the adjusting pressure passage 256 to the atmospheric pressureintroducing port 258. Accordingly, air flows through the atmosphericpressure introducing port 258 only when a pressure higher than apressure in the first pressure changing chamber 242 is generated in theadjusting pressure passage 256.

A booster piston 262 is fit in the center of the diaphragm 236. Thebooster piston 262 is slidably supported by the rear shell 232 so thatan end thereof is exposed in the second pressure-changing chamber 244.Additionally, the booster piston 262 is urged toward an originalposition, that is, in a rightward direction in FIG. 7, by a spring 263provided within the second pressure-changing chamber 244.

An inner space 264 is formed in a center of the booster piston 262, theinner space extending in a radial direction of the booster piston 262.Additionally, the booster piston 262 is provided with a negativepressure passage which connects the second pressure changing chamber 244to the internal space 264 and a pressure changing passage 268 whichconnects the internal space 264 and the first pressure changing chamber242.

The internal space 264 of the booster piston 262 is provided with apressing force transmitting member 270 which is slidable in an axialdirection thereof. The pressing force transmitting member 210 has anannular air valve 272 on an end located on a rearward side of thevehicle, and has a cylindrical pressing force transmitting part 274 onan end located on a forward side of the vehicle.

A control valve 276 is provided in the internal space 264 of the boosterpiston 262. The control valve 276 includes a cylindrical part 278 fixedon an inner wall of the internal space 264 and a flat part 280 formed onan end located on a forward side of the vehicle. The flat portion 280can move inside the inner space 264 in an axial direction of the controlvalve 276 with elongation and compression of the cylinder part 278.

A through hole 282 is formed in the flat portion 280 of the controlvalve 276, the through hole 282 extending in the center of the flatportion 280. An input rod 284 is inserted into the through hole 282. Thediameter of the through hole 282 is sufficiently larger than thediameter of the input rod 284. Thus, an appropriate clearance is formedbetween the periphery of the input rod 284 and the through hole 282.

An end of the input rod 284 located on the forward side of the vehicleis connected to the pressing force transmitting member 270, and theother end of the input rod 284 located on the rearward side of thevehicle is connected to the brake pedal shown in FIG. 6. An end of aspring 286 is engaged with the input rod 284. The other end of thespring 286 is engaged with the cylindrical part 278 of the control valve276. The spring 286 urges the input rod 284 and the pressing forcetransmitting member 270 toward the brake pedal 202 relative to thecylindrical part 278, that is, the booster 262. When a brake pressingforce is not input to the input rod 284, the input rod 284 and thepressing force transmitting member 270 are held at a reference pointshown in FIG. 1 by the above-mentioned urging force generated by thespring 286.

An end of a spring 288 is also engaged with the input rod 284. The otherend of the spring 288 contacts the flat part 280 of the control valve276. An urging force of the spring 288 serves as a force to urge theflat part 280 toward the air valve 272.

When the pressing force transmitting member 270 is held at the referenceposition as shown in FIG. 6, no force against the urging force of thespring 288 is exerted on the flat portion except for a reaction forcegenerated by the air valve 272. Accordingly, when the pressing forcetransmitting member 270 is located at the reference point, the flat part280 is maintained to be in contact with the air valve 272. The diameterof the air valve 272 is set to be larger than the diameter of thethrough hole 282 of the control valve 276. Accordingly, under such acondition, a state in which the through hole 282 is closed by the airvalve 272 is established.

The booster piston 262 is provided with an annular valve seat 290 at aposition opposite to the flat part 280 of the control valve 276. Thevalve seat 290 is formed so that a predetermined clearance is maintainedbetween the valve seat 290 and the flat part 280 when the input rod 284and the pressing force transmitting member 270 are located at thereference position. If there is a clearance between the valve seat 290and the flat part 280, the above-mentioned negative pressure passage 266communicates with the internal space 264. Additionally, if the valveseat 290 contacts the flat portion 280, the negative pressure passage266 is disconnected from the internal space 264.

Air filters 292 and 294 are provided in the internal space 264 of thebooster piston 262. The internal space 264 is open to an atmosphericspace via the filters 292 and 294. Accordingly, an atmospheric pressureis always introduced around the through hole 282 of the control valve276.

The booster piston 262 contacts a reaction disc 296 at an end surfacelocated on the forward side of the vehicle. The reaction disc 296 is adisc-like; member formed by an elastic material. The other surface ofthe reaction disc 296 contacts an output rod 298. The output rod 298 isa member which is connected to an input shaft of the master cylinder 206shown in FIG. 5. When a brake pressing force is exerted on the brakepedal 202, a pressing force corresponding to the brake pressing force istransmitted to the master cylinder via the output rod 298. On the otherhand, a reaction force corresponding to the master cylinder pressurePM/C is input to the reaction disc 296.

The center of the reaction disc 296 is opposite to the pressing forcetransmitting part 274 of the pressing force transmitting member 270. Thepressing force transmitting member 270 is formed so that a predeterminedclearance is formed between the pressing force transmitting part 274 andthe reaction disc 296 when the pressing force transmission member 270 isLocated at the reference position with respect to the booster piston262.

A description will now be given of an operation of the vacuum booster204 and an operation of the brake force control apparatus according tothe present embodiment. In the present embodiment, similar to the ECU 10of the above-mentioned first embodiment, the ECU 200 determines whetherthe brake assist control should be started in accordance with the mastercylinder pressure PM/C and the rate of change ΔPM/C of the mastercylinder pressure PM/C.

That is, the ECU 200 continues the normal control when the mastercylinder pressure PM/C and the rate of change ΔPM/C of the mastercylinder pressure PM/C detected by the hydraulic pressure sensor 212 donot satisfy the predetermined start condition. On the other hand, theECU 200 starts the brake assist control when PM/C and ΔPM/C thereofsatisfy the start condition.

In the system according to the present embodiment, when the ECU 200performs the normal control both the negative pressure introducing valve252 and the atmospheric pressure introducing valve 254 are maintained tobe in the OFF state. In this case, a negative pressure is introducedinto the negative pressure chamber 240 of the vacuum booster 204, and anegative pressure is also introduced into the second pressure-changingchamber 244. A description will now be given of an operation of thevacuum booster 204 under such a condition.

When the brake pressing force FP is riot applied to the brake pedal 202,the input rod 284 and the pressing force transmitting member 270 areheld at the reference position (position shown in FIG. 6). In thiscease, a state in which the air valve 272 is seated on the flat part 280of the control valve 276, and the flat part 280 is separated from thevalve seat 290, that is, a state in which the pressure changing passage268 is disconnected from the atmospheric space and communicates with thenegative pressure passage 266, is formed.

Under such a condition, the second pressure-changing chamber 244communicates with the first pressure-changing chamber 242. Accordingly,a pressure inside the first pressure-changing chamber becomes a negativepressure similar to the pressure inside the second pressure changingchamber 244 and the pressure inside the negative pressure chamber 240.When the pressure inside the first pressure-changing chamber 242 isequal to the pressure inside the second pressure-changing chamber 244,no force caused by the negative pressures is exerted on the diaphragm236. Therefore, when the brake pressing force FP is not input, apressing force is not transmitted from the output rod 298 to the mastercylinder 206.

When the brake pressing force FP is applied to the brake pedal 202, theinput rod 284 is moved relative to the booster piston 262 in the forwarddirection of the vehicle, that is, in the rightward direction in FIG. 6.When a relative displacement of the input rod 284 reaches apredetermined length, an end surface of the pressing force transmittingpart 274 contacts the reaction disc 296, and the flat part 280 of thecontrol valve 276 sits on the valve seat 290 of the booster piston 262so that the negative pressure passage 266 is disconnected from thepressure changing passage 268.

If the input rod 284 is further pressed in the direction toward thereaction disc 296, the input rod 284 and the pressing force transmittingmember 270 continues to move while elastically deforming the center partof the reaction disc 296, that is, a part of the reaction disc 296(hereinafter, simply referred to as a center part) which contacts thepressing force transmitting part 274. If the relative displacement ofthe pressing force transmitting member 270 is increased as mentionedabove, a reaction force corresponding to an elastic deformation, thatis, an elastic force corresponding to the brake pressing force FP istransmitted to the input rod 284.

Additionally, after a state in which the flat part 280 is seated on thevalve seat 290 is established as mentioned above, the displacement ofthe flat part 280 relative to the booster piston 262 is restricted.Thus, if the input rod 284 is further pressed in the direction towardthe reaction disc 296 after such a condition is established, the airvalve 272 is separated from the flat part 280 of the control valve 276,and the pressure changing passage 268 communicates with the through hole282.

If such a state is established, an atmospheric air is introduced intothe first pressure-changing chamber 242 via the through hole 282 and thepressure changing passage 268. As a result, the pressure inside thefirst pressure-changing chamber 242 becomes higher than the pressureinside the second pressure-changing chamber 244 and the negativepressure chamber 240. As mentioned above, if a pressure difference ΔPBis generated between the first pressure changing chamber 242 and each ofthe second pressure changing chamber 244 and the negative pressurechamber 240, a pressing force FA (hereinafter, referred to as brakeassist force FA) which urges the diaphragm 236 in a direction toward thefront of the vehicle is exerted on the diaphragm 236.

It should be noted that the brake assist force FA can be approximatelyrepresented by the following equation by using an effectivecross-sectional area SB of the negative pressure chamber 240 and aneffective cross-sectional area SC of the second pressure changingchamber 244.

    FA=(SB+SC)·ΔPB                              (2)

The thus-generated brake assist force FA is transmitted from thediaphragm 236 to the booster piston 262, and further transmitted to aperiphery of the reaction disc 296, that is, a part of the reaction disc(hereinafter, simply referred to as a peripheral part) which contactsthe booster piston 262.

When the brake assist force FA is input from the booster piston to theperipheral part of the reaction disc 296, an elastic deformation isgenerated in the peripheral part of the reaction disc 296. This elasticdeformation increases as a pressure difference ΔP between opposite sidesof the diaphragm 236 increases, that is, as the introduction of air intothe first pressure changing chamber 242 is continued.

In the process in which an amount of elastic deformation in theperipheral part of the reaction disc 296 is increased as mentionedabove, the booster piston is moved relative to a reaction forcetransmitting part 28 in the direction toward the front of the vehicle.Then, if the amount of elastic deformation of the peripheral part of thereaction disc 296 reaches a value almost equal to the amount of elasticdeformation of the center part of the reaction disc 296, the flat part280 of the control valve 276 contacts the air valve 272, and theintroduction of atmospheric air to the first pressure changing chamberis stopped.

As a result, the pressure difference ΔP generated between opposite sidesof the diaphragm 236 is adjusted to a value corresponding to the brakeforce FP input to the input rod 284. Additionally, the brake assistforce FA=(SB+SC)·ΔPB becomes a value corresponding to the brake pressingforce FP. At this time, a resultant force of the brake assist force FAand the brake pressing force FP is transmitted to the master cylinder206.

When the resultant force of the brake assist force FA and the brakepressing force FP is transmitted to the master cylinder 206, the mastercylinder 206 generates a master cylinder pressure PM/C having apredetermined power ratio with respect to the brake pressing force FP.

The ECU 200 turns off the SH 216 and SR 220 so as to set the hydrauliccircuit connected to the master cylinder 206 to a normal state. When thehydraulic circuit is set to the normal state, the master cylinderpressure PM/C is introduced into the wheel cylinder 218 as it is.Accordingly, the brake force generated in the wheel cylinder 218 isadjusted to a level corresponding to the brake pressing force FP.

If a slip rate S of a wheel exceeds a predetermined value after thebraking operation is started, the ECU 200 starts the ABS control similarto the ECU 10 of the above-mentioned first embodiment. The ABS controlis performed when the brake pedal 202 is pressed, that is, when themaster cylinder pressure PM/C is appropriately increased.

Under the condition in which the master cylinder pressure PM/C isappropriately increased, the SH 216 is set to the open state and the SR220 is set to the closed state, and, thereby, the wheel cylinderpressure PW/C is increased with the master cylinder pressure PM/C as anupper limit value. Hereinafter, this state is referred to as apressure-increasing mode 1. Additionally, the wheel cylinder pressurePW/C is maintained without being increased or decreased by the SH 216being set to the closed state and the SR 220 being set to the closedstate. Hereinafter, this state is referred to as a holding mode 2.Further, the wheel cylinder pressure PW/C is decreased by the SH 216being set to the closed state and the SR 220 being set to the openstate. Hereinafter, this state is referred to as a pressure decreasingmode 3. The ECU 200 achieves, if necessary, the above-mentionedpressure-increasing mode 1, holding mode 2 and pressure-decreasing mode3 so that a slip rate S of the wheel becomes an appropriate value.

When a depression of the brake pedal 202 is released by the driverduring execution of the ABS control, the wheel cylinder pressure PW/Cmust be immediately decreased. In the system according to the presentembodiment, the check valve 222 is provided in the hydraulic circuitcorresponding to the wheel cylinder 218. The check valve 222 permits afluid flow only in the direction from the wheel cylinder 218 to themaster cylinder 206. Thus, according to the system of the presentembodiment, the wheel cylinder pressure PW/C of the wheel cylinder 222can be immediately decreased after the depression of the brake pedal 202is released.

In the system according to the present embodiment, when the ABS controlis performed, the wheel cylinder pressure PW/C is increased by themaster cylinder 206 as a fluid pressure source. Additionally, the wheelcylinder pressure PW/C is decreased by having the brake fluid in thewheel cylinder flow to the reservoir 224. Accordingly, if thepressure-increasing mode and the pressure-decreasing mode are repeatedlyperformed, the brake fluid in the master cylinder 206 gradually flows tothe reservoir 224.

However, in the system according to the present embodiment, the brakefluid in the reservoir 224 is delivered to the master cylinder 206 bythe pump 226. Thus, if the ABS control is continued for a long time, aso-called bottoming of the master cylinder does not occur.

A description will now be given of an operation achieved by the ECU 200performing the brake assist control. As mentioned above, when the mastercylinder pressure PM/C and the rate of change ΔPM/C thereof satisfy thepredetermined start condition, the ECU 200 starts the brake assistcontrol. The brake assist control is achieved by turning on both thenegative pressure introducing valve 252 and the atmospheric pressureintroducing valve 254, that is, by closing the negative pressureintroducing valve 252 and opening the atmospheric pressure introducingvalve 254.

The ECU 200 maintains both the negative pressure introducing valve 252and the atmospheric pressure introducing valve 254 to be set to the OFFstate until the ECU 200 determines that the start condition of the brakeassist control is established after the brake pedal 202 is pressed.Then, if it is determined that the start condition is established, boththe negative pressure introducing valve 252 and the atmospheric pressureintroducing valve 254 are set to the ON state.

Until both the negative pressure-introducing valve 252 and theatmospheric pressure introducing valve 254 are set to the ON state, theinput rod 284 moves prior to the booster piston 262. As a result, thecontrol valve 276 sits on the valve seat 290 and the air valve 272separates from the control valve 276. Thereby, atmospheric air isintroduced into the first pressure changing chamber 242, and the brakeassist force FA=(SB+SC)·ΔPB is generated.

Under such a condition, if the negative pressure introducing valve 252and the atmospheric pressure introducing valve 254 are set to the ONstate, a pressure inside the first pressure changing chamber 242 and thesecond pressure changing chamber 244 is rapidly increased to anatmospheric pressure. As a result, a pressure difference ΔPAIR isgenerated between the negative pressure chamber 240 and the firstpressure changing chamber 242. In this case, a brake assist force FArepresented by the following equation is exerted on the diaphragm 236.

    FA=SB·ΔPAIR                                 (3)

The brake assist force FA is transmitted from the diaphragm 236 to thebooster piston 262, and further transmitted to the peripheral part ofthe reaction disc 296. Additionally, the brake pressing force FP whichis exerted on the brake pedal 202 is also transmitted to the reactiondisc 296. Accordingly, thereafter, a resultant force of the brake assistforce FA and the brake pressing force FP is transmitted to the mastercylinder 206.

In the system according to the present embodiment, similar to theabove-mentioned first embodiment, the brake assist control is startedwhen the brake pressing force FP is not sufficiently increased, that is,under a condition in which a large brake assist force FA has not beenobtained. Accordingly, the brake assist force FA exerted on the boosterpiston 262 shows a sharp increase before or after the brake assistcontrol is started.

If the sharp change occurs in the brake assist force FA as mentionedabove, the booster piston 262 is rapidly and relatively moved toward thefront of the vehicle immediately after the brake assist control isstarted. Then, when such a sharp change is generated in the boosterpiston 262, a phenomenon occurs in which the control valve 276 which wasseated on the valve seat 290 before the brake assist control was startedis separated from the valve seat 290 when the control is started.

When the control valve 276 is separated from the valve seat 290, thesecond pressure changing chamber 244 communicates with the firstpressure changing chamber 242. Accordingly, if a negative pressure isstored in the second pressure changing chamber 244, the negativepressure is provided from the second pressure changing chamber 244 tothe first pressure changing chamber 242 after the brake assist controlis started. As a result, there is a problem in that the brake assistforce FA cannot be raised immediately.

However, in the vacuum booster 204 of the present embodiment,atmospheric air is introduced into the second pressure-changing chamber244 at the same time the brake assist control is started. Thus,according to the system of the present embodiment, if the phenomenon inwhich the control valve 276 is separated from the valve seat 290 afterthe brake assist control is started occurs, the brake assist force FAcan be raised immediately.

The ECU 200 sets the hydraulic circuit connected to the master cylinder216 to a normal state after the execution condition of the brake assistcontrol is established and until the execution condition of the ABScontrol is established. In this case, the master cylinder pressure PM/Cis introduced to the wheel cylinder 218 without change. Accordingly, thewheel cylinder pressure PW/C is rapidly increased from a pressurecorresponding to "(SB+SC)·ΔPB+FP" to a pressure corresponding to"SB·ΔPAIR+FP" when the brake assist control is started.

As mentioned above, according to the system of the present embodiment,when an emergency braking operation is performed, the wheel cylinderpressure PW/C can be rapidly increased to a value sufficiently largerthan the brake pressing force FP. Thus, according to the system of thepresent embodiment, a large brake force can be generated immediatelyafter establishment of a condition in which an emergency braking isrequired is established even if the driver is a beginner-grade driver.

After the wheel cylinder pressure PW/C is rapidly increased as mentionedabove, the slip rate S of the wheel is rapidly increased, and finallythe execution condition of the ABS control is established. After theexecution condition of the ABS control is established, the ECU 200 sets,if necessary, the above-mentioned pressure increasing mode 1, holdingmode 2 and pressure decreasing mode 3 so that a slip rate S of the wheelbecomes an appropriate value.

In the system according to the present embodiment, in a period duringwhich the brake pressing force FP is applied to the brake pedal 202after the brake assist control is started, the master cylinder pressurePM/C is maintained to be a pressure corresponding to "SB·ΔPAIR+FP".Accordingly, by monitoring the output signal of the master cylinderpressure PM/C detected by the hydraulic pressure sensor 212, the ECU 200can determine whether or not the depression of the brake pedal 202 isreleased by more than a predetermined amount. Upon detection of therelease of the depression of the brake pedal 202 by more than thepredetermined amount, the ECU 200 stops supply of the drive signals tothe negative pressure introducing valve 252 and the atmospheric pressureintroducing valve 254, and terminates the brake assist control.

In the above-mentioned second embodiment, as mentioned above, the mastercylinder pressure PM/C is discontinuously decreased from a pressurecorresponding to "SB·ΔPAIR+FP" to a pressure corresponding to"(SB+SC)·ΔPB+FP" when the brake assist control is terminated. Thereby,the change in the master cylinder pressure PM/C accompanies a vibration.Accordingly, similar to the above-mentioned first embodiment, it ispossible that the master cylinder pressure PM/C and the rate of changeΔPM/C of the master cylinder pressure PM/C satisfy the start conditionof the brake assist control immediately after the brake assist controlis terminated. In the present embodiment, the brake assist control isprevented from being improperly started, immediately after the brakeassist control is terminated, by the ECU 200 performing the controlroutine shown in FIG. 4 similar to the case of the above-mentioned firstembodiment.

It should be noted that, in the above-mentioned second embodiment,although the master cylinder pressure PM/C is used as the basicparameter for discriminating a normal braking operation and an emergencybraking operation, the basic parameter is not limited to this, and,similar to the first embodiment, the brake pressing force FP, the pedalstroke L, the vehicle deceleration G, the assumed vehicle speed VSO orthe vehicle speed VW** may be used as the basic parameter.

The present invention is not limited to the specifically disclosedembodiments, and variations and modifications may be made withoutdeparting from the scope of the present invention.

What is claimed is:
 1. A brake force control apparatus of a vehicleselectively performing a normal control for generating a brake forcecorresponding to a brake pressing force and a brake assist control forgenerating a brake force larger than that of the normalcontrol,characterized by: control start determining means fordetermining (step 108) whether or not the brake assist control should beperformed based on a change in a driving condition of said vehicle dueto an operation of a brake pedal (30; 202); and control prohibitingmeans for prohibiting (step 106, 102) an execution of a subsequent brakeassist control operation after a previous brake assist control operationwas terminated and until a predetermined time has been passed.
 2. Thebrake control apparatus as claimed in claim 1, characterized in thatsaid control start determining means determines whether or not the brakeassist control should be performed based on a pressure (PM/C) of amaster cylinder (32; 206).
 3. The brake control apparatus as claimed inclaim 2, characterized in that said control start determining meansprohibits (step 100, 102) the brake assist control when the pressure(PM/C) of the master cylinder (32; 206) is greater than a predeterminedvalue.
 4. The brake control apparatus as claimed in claim 2,characterized in that said control start determining means prohibits(step 104, 102) the brake assist control when a rate of change (ΔPM/C)of the pressure (PM/C) of the master cylinder (32; 206) is greater thana predetermined value.
 5. The brake control apparatus as claimed inclaim 1, characterized in that said predetermined time ranges from 100milliseconds to 200 milliseconds.
 6. The brake control apparatus asclaimed in claim 1, characterized in that said predetermined time is 120milliseconds.
 7. The brake control apparatus as claimed in claim 1,characterized in that said control start determining means determineswhether or not the brake assist control should be performed based on abrake pressing force (FP).
 8. The brake control apparatus as claimed inclaim 1, characterized in that said control start determining meansdetermines whether or not the brake assist control should be performedbased on a pedal stroke (L) of the brake pedal (30; 202).
 9. The brakecontrol apparatus as claimed in claim 1, characterized in that saidcontrol start determining means determines whether or not the brakeassist control should be performed based on a deceleration (G) of saidvehicle.
 10. The brake control apparatus as claimed in claim 1,characterized in that said control start determining means determineswhether or not the brake assist control should be performed based on aspeed (VSO) of said vehicle.
 11. The brake control apparatus as claimedin claim 1, characterized in that said control start determining meansdetermines whether or not the brake assist control should be performedbased on a wheel speed (VW**) of said vehicle.